Thank you for your reply, sir. I found out that there's already a
plat_mem_setup defined in vr41xx/common/init.c that's why I cannot
rename the function invoked by early_initcall to plat_mem_setup.

Actually,
when I examined it further, I found out that using arch_initcall works
just fine except that the add_memory_region is not called before
everything else. It causes error because there's no memory region
allotted.